i am designing a narrow band pass filter using ua741. when i simulate the circuit on pspice i get exactly what i want.but after implementation i cant vary frequency of the signal by varying resistance.it only varies amplitude of the signal.now what is the problem here i can't figure it out.my circuiis this:

 

To keep the filter cutoff frequency, you cannot vary just one component.

Put a pot between Vout and gnd as a voltage divider. That way the filter cutoff is not effected by adjusting gain...

 

thank you.what range of pot should i use?amd if i use pot then can i vary frequency of signal?please let me know

 

what range of pot should i use?amd if i use pot then can i vary frequency of signal?

Are you trying to make the filter tune-able by varying either the resistances or the capacitors?

Selecting a pot to adjust the amplitude of a signal requires knowing the input impedance of the following stage, and to a lesser extent, the output impedance of the upstream stage...

 

thank you.what range of pot should i use?amd if i use pot then can i vary frequency of signal?please let me know

You cannot vary the bandpass frequency of such a filter with a single pot. All three resistors need to vary.
Your simulation will show you that.

 

Are you trying to make the filter tune-able by varying either the resistances or the capacitors?

Selecting a pot to adjust the amplitude of a signal requires knowing the input impedance of the following stage, and to a lesser extent, the output impedance of the upstream stage...

i want 2 vary the frequency of the signal but by varying pot it only varies amplitude.i did simulation and i got exact result i wanted but in practical i can't

 

You cannot vary the bandpass frequency of such a filter with a single pot. All three resistors need to vary.
Your simulation will show you that.

i did simulation and by varying R2 i can vary the cutoff frequency.i did ac analysis.but after implementation it doesn't work.now what is the problem i can't figure it out.please hep me

 

Post the circuit you simulated with values and power supplies.

 

Post the circuit you simulated with values and power supplies.

here i applied 4k sinewave signal at input and designed to get this value.during simulation i got the exact value but in lab it doesn't vary frequency.plz help me.now what to do.or wht this doesn't work in lab?

 

We cannot read the values of the R and Cs.

You seem not to understand this very important point:
The MFB filter you show is not tuneable by changing just one resistor, or just one capacitor. Changing just one resistor, or one capacitor in isolation changes the center frequency, and the shape of the filter. This likely is not useful or desirable...

In order to shift the center frequency of the filter, keeping the bandwidth and other attributes, you must change all three resistors at the same time. (or change the two capacitors at the same time).

 

We cannot read the values of the R and Cs.

You seem not to understand this very important point:
The MFB filter you show is not tuneable by changing just one resistor, or just one capacitor. Changing just one resistor, or one capacitor in isolation changes the center frequency, and the shape of the filter. This likely is not useful or desirable...

In order to shift the center frequency of the filter, keeping the bandwidth and other attributes, you must change all three resistors at the same time. (or change the two capacitors at the same time).

thank you very much.i did this cause i got this circuit from a link and there was instruction about this circuit to change only R2. that's why i did this.will u can plz help me by showing me a good circuit diagram of narrow band pass filter range of 1kHz.plzzzzz

 

..help me by showing me a good circuit diagram of narrow band pass filter range of 1kHz.

Fixed tuned to 1KHz? Read this app note (see page 6)

Tunable 1KHz +- small range of frequencies using pot to vary two resistances simultaneously.

 

Fixed tuned to 1KHz? Read this app note (see page 6)

Tunable 1KHz +- small range of frequencies using pot to vary two resistances simultaneously.

thanks.not fixed but i have to tune in between 1kHz.

 

can anyone plz help me by informing me why ua741 doesn't give exact results.i have to give answer to my teacher tomorrow but i don't know the answer.plz plz plz inform me

 

thanks.not fixed but i have to tune in between 1kHz.

can anyone plz help me by informing me why ua741 doesn't give exact results....

You will have to explain what the shortcoming is...

 

You will have to explain what the shortcoming is...

sorry i didn't understand what u said.i can't find the problems with my circuits.but i have to explain it to my teacher.this is a portion of my thesis.so plz plz plz help me.what should i say about the problems of my circuit.plz .

 

But what, exactly, is the problem?

 

problem is i can't vary cut off frequency

 

problem is i can't vary cut off frequency

I give up!